Dehydration (thirst, dark urine, confusion, cramps, feeling weak) 

Heat exhaustion (headaches, dizziness, nausea, fast pulse). If you feel unwell, lie down in a cool room, drink plenty of water and cool yourself with a splash of water 

Heat exhaustion could lead to heatstroke - symptoms are falling unconscious and seizures. Call 999 immediately if you think someone might have heatstroke